This invention relates to subsea well drilling systems, and more particularly to permanent guide bases for such systems.
When drilling for oil and gas wells from a floating drilling rig, a permanent guide structure is attached to a 30 inch wellhead housing which is mounted on a 30 inch conductor pipe extending down into the wellbore. The guide structure usually has four upstanding posts on a six foot radius, 90 degrees apart. The posts are usually 8 5/8 inches in diameter and might be five or more feet tall, the height depending on the other subsea equipment that will have to mate with the wellhead housing during the drilling and completion of the subsea well. Guide lines, which extend from the posts back to the floating drilling rig, are used to guide the subsea equipment packages to and from the subsea well site and the wellbore. The inside of the center section of the guide structure is designed to accept and trap the subsea 30 inch wellhead housing which has a standard internal preparation to accept a variety of smaller wellhead housing sizes such as those with 21 3/4 inch, 18 3/4 inch, or 16 3/4 inch bores. The inside of the center section of the guide structure also has a landing shoulder to support the housing and a lock means, such as a bolted flange, for trapping the wellhead housing in the center section of the guide structure. Usually the 30 inch wellhead housing has an external slot down its side, and the bore of the center section of the guide structure has an orientation pin in order to align outlets in the housing with outlets built into the guide structure.
The 30 inch wellhead housing is landed in and attached to the guide structure, and the housing (with attached guide structure) is then run to the ocean floor and landed on an "upward" facing funnel of a temporary guide base which is resting on the ocean floor. Either bolted or welded to the bottom of the guide structure is a spherical rib section which permits the guide structure to gimbal with respect to the funnel on the temporary guide base. Sometimes the ocean floor is not level, and the gimbal capability permits the guide structure and wellhead housing with conductor pipe to align itself with the vertical/nearly vertical well bore, so that it can be cemented into position. Some designs did not provide for removal or recovery or replacement of the guide structure or its spherical rib gimbal section and still leave the wellhead housings in place. With those designs, if the guide structure had to be recovered, recovery of the wellhead housings as well was required. If a guide structure was damaged, there was no way to repair or change it. If mechanical damage occurs to a post on a guide structure, many guide structure designs permit replacement of a post, some by divers but others without the aid of divers. Since four posts are provided on guide structures, one might be omitted and guidance of equipment onto or off of the wellhead housing can still be accomplished. In some cases, the damaged post might be the key post for Xmas Tree installation or flow line pull-in and attachment, and if two or more posts or a key post are damaged, then the well might have to be abandoned.
Subsea wells are very expensive, usually tens of millions of dollars each, so that abandonment can be an expensive loss. Sometimes an exploratory well may be drilled through a more basic guide structure, but if good production is found it may be desireable to convert to a production well. The removal of the basic guide structure and its replacement with a production guide structure having flowline pull-in and attachment capability might save a lot of money and permit quicker cash flow from a discovery well. Furthermore, in some cases the operator wants to recover and reuse the wellhead equipment as well as the guide structure, gimbal, and temporary base, all of which requires a means to disconnect this equipment from the wellhead housing.